U.S. Pat. No. 5,213,374, which I incorporate herein by reference, discloses a coupling ring for joining circular air ducts. While the ring is enjoying increasing commercial success, I have discovered that variations in the actual diameter of the ducts to be connected may vary sufficiently from the nominal diameter that the workmen may have difficulty inserting the duct ends into the oppositely opening grooves of the coupling ring. Thus a coupling ring for an 18" duct may not, on occasion, fit properly with nominal 18" round ducts, or a coupling ring for 30" duct may not always fit with nominal 30" round ducts. While insistence that the duct manufacturers hold more closely the allowable tolerances on the duct diameters may be helpful, I have found that by a modification of the coupling ring itself, I am able to accommodate the variations being experienced in the duct diameters.